Although epoxy resin has been industrially used in a wide range of areas, performances for epoxy resin have been more and more highly required in recent years along with the development of industry. For example, epoxy resins have been conventionally used for a copper-clad laminated plate for electronics parts and electronics devices, and a sealing agent etc. to form a semiconductor package. In order to enhance the credibility of thermal shock, it is required that the Tg. (glass-transition temperature) of a cured material of the epoxy resin should be more highly improved. In addition, from the viewpoint of taking safety into consideration such as a fire prevention and a fire spread retardation, and also from the viewpoint of coping with an environmental problem, the epoxy resin, which has a flame-retardant effect without using the conventionally used halogen, has been actively developed.
For example, as for the production of a prepreg for manufacturing a multilayer laminated plate, the prepreg having the improved Tg is proposed which is obtained by adding a solid melamine having three amino groups in an epoxy resin composition (Patent document 1). However, in cases where the crosslink density of the cured epoxy resin material is improved to raise the Tg, by incorporating the melamine that is a trifunctional material in this way. there was a disadvantage in that molding properties of prepreg are poor, for the reason that the dispersing properties of the melamine, when the prepreg is manufactured, is poor since the melamine is solid.
In addition, a one-component epoxy resin composition using a trifunctional epoxy resin (having three epoxy groups) is proposed as a filler for plugging a through hole of a printed wiring substrate (Patent document 2). In this case, a large amount of inorganic filler required for decreasing the linear expansion coefficient of cured material can be added if a liquid low-viscosity epoxy resin is used, and workability can be excellent, however, the epoxy resin cured material having a satisfactory Tg could not be obtained.
Moreover, all of the aforementioned epoxy resins had unsatisfying evaluations concerning flame retardancy. On the other hand, as a means of improving the flame retardancy of epoxy resin, a method for adding phosphate esters in the epoxy resin is also known (Patent document 3). The flame retardancy of the epoxy resin is improved to some extent by this method. However, since the added phosphate ester does not have reactive properties with other components in the epoxy resin, there was a disadvantage in that a bleed phenomenon in which a part of the phosphate ester is liberated when the epoxy resin is cured occurs, or the Tg of a cured material markedly drops.